Diazonium and photopolymerizable compositions have been used in photosensitive elements, such as lithographic offset printing plates, for a number of years. In the field of lithographic offset printing, there are known many processes for producing an image on a substrate by exposure to U.V. radiation via use of such photopolymerizable compositions and thereafter developing the image. One of the earlier methods is the additive process whereby the image is reinforced with an additive lacquer. Although useful for many years, the image produced by such a process currently has limited application as it does not have desirable wear and chemical resistance properties; the quality of the image is oftentimes only fair; and the developer needed to produce such an image employs solvents which are toxic and hazardous to the environment.
A presensitized lithographic printing plate is described in U.S. Pat. No. 3,136,637, which employs an organophilic, hydrophobic resin overlaying a light sensitive diazo layer which upon exposure becomes insolubilized, and which in turn is coated on a dimensionally stable hydrophilic surface base sheet which is typically metal. This process is inherently undesirable in that the exposed image, although hardened and adherent to the underlying diazo base layer, is prone to attack and partial removal during development. The overlying resin image is also prone to attack by pressroom solvents such as blanket washes and common cleaners for printing plates.
A presensitized printing plate having a dual layer coating of an intermediate sensitized diazo layer on a base plate on top of which is a photopolymerizable prepolymer resist composition is described in U.S. Pat. No. 3,462,267. The prepolymer is an aryl alkyl ester which requires relatively long exposure times, and must be developed by toxic and hazardous solvents such as xylene, methylethylketone and trichloroethane.
U.S. Pat. No. 3,808,004 describes a lithographic plate employing a photopolymerizable coating over a diazo base resin. Conventional polymerizable resins such as polyvinyl cinnamate available from Eastman Kodak are employed and exposed plates are developed with commercial additive lacquers. These plates suffer, however, from poor shelf life and toxic and hazardous solvents used in their development.
U.S. Pat. No. 3,905,815 discloses a lithographic printing plate comprising a hydrophilic base-sheet which bears a coating of a light-sensitive diazo resin, over which is coated a nontacky, soluble, photopolymerizable layer. The photopolymerizable layer comprises a film forming binder having dispersed therethrough one or more polymerizable monomers and a photoinitiator system. These compositions are, however, susceptible to overdevelopment, as the film forming binders are not polymerizable.
U.S. Pat. No. 4,233,390 describes a lithographic printing plate comprised of two layers; one water soluble diazo-based layer having coated thereon a second photopolymerizable layer comprised of a photopolymerizable monomer or oligomer, a photoinitiator, and various non-polymerizable resins. In addition to using toxic photoinitiators, for example, Michler's Ketone, and requiring additional and expensive non-polymerizable resins, this composition also requires undesirably high coating weights.
A photopolymerizable printing plate employing a polymerizable composition comprising a specific solvent-soluble diazo composition on a hydrophilic surfaced substrate is described in U.S. Pat. No. 4,886,731. The polymerizable coating is made up of three layers: a first layer comprising a water-soluble diazo compound; a second layer containing a solvent-soluble diazo compound and a trifunctinal acrylate monomer; and a top coat layer containing photoinitiators, functional acrylate oligomer, the trifunctional acrylate monomer, a polymer binder containing a pigment, and the solvent-soluble diazo compound employed in the second layer. The application of the separate coatings on one another in the necessary amounts renders such a system commercially undesirable.
It is therefore an object of the present invention to provide a photopolymerizable coating composition useful in producing lithographic offset printing plates which avoids the shortcomings and disadvantages of conventional printing systems.
It is also an object of this invention to provide a method for the production of such a lithographic offset printing plate and method for its use.